Thanks to everyone for suggestions on how to work around the problem of not
being able to send to lists as postmaster.  I think we will be able to
manage new list requests with a ListManager account by having the requests
sent to an IMAP account to which all of the postmaster will have access.

One other question, though.  The documentation refers to the forbidden
addresses as "MAILER, POSTMASTER, ROOT, UUCP, et al."  What are the others
included in et al?  We have a number of automated e-mail processes, some of
which come from root on Unix machines.  We also have some PCs that send mail
as "administrator@whatever".  Is administrator another forbidden address?